While I think Scarlet Witch is an old standby, the truth is, she's kind of a supporting character in her world. Pietro is usually the lead, and a film would need to subvert that, in addition to making a new world for her since mutancy and all it's trappings are unavailable. I'd be interested in making such a thing, not sure I'd like anyone else's take on it though. It would be new though, and I think feature the kind of Not!Magic we've seen... I also think they won't do it for fear of stepping on Dr. Strange's mystical toes.

Quicksilver has always played second fiddle to Wanda. There's a reason she's fairly popular in the fan community, and he's... not. He's just always kinda there, being a prick, whereas she developed a ton of friendships and relationships with others and evolved as a character in pretty significant ways.

The problem with a SW movie is that most of her solo material from the comics is convoluted nonsense, so they'd basically have to start from scratch for a movie. Which is certainly doable, but I don't know if they'd do it